Monday, April 1, 2013

March 30, 2013: Hubbard Brook...again.

Almost a year ago, we made our first concerted effort at re-building the underpinnings of a long board walk between Shays' Rebellion and Hubbard Brook in Sheffield. For your review, here's the Link to that post.

Still passable, but treacherous

Here we are again, fixing another section with the same problem. Why didn't we fix the whole thing a year ago? Because it wasn't broken, yet. Seriously, we think we have the walkway pretty well stabilized at this point, the remaining sections have all been addressed, at least for the next couple of seasons.

This walkway was installed in the late 80's/early 90's when the NPS purchased land in the Berkshires to help reduces the number of AT miles on roads. Of course most of the land that was available was not suitable for houses or agriculture--so basically we got the swamps.

To build the initial walkway, the 10ft x 4ft platforms that make it up were supported in helical piles that look like this:

Each 10ft section is supported by a pair of these

They are about 4ft long and installed by slipping a large rock bar through the eye at the top and turning them into the ground like a giant screw.

Back then, AT maintainers did not have experience building large boardwalks such as are now found at Pawling NY, Pochuck NJ, or Thundering Falls VT. These modern ones are designed to be universally accessible portions of the AT, capable of supporting powered wheelchairs, and large numbers of hikers at once. Their piles are much larger, and must be driven in by a machine. Some of the piles at Thundering Falls were driven 90ft down before they were solid enough to support that boardwalk.

Our installation at Hubbard Brook was not intended to meet this type of structural load--over time, the smaller piles we installed 20 years ago have begun to tilt sideways as the ground freezes and thaws.

To provide better support, we've constructed cribs:

Completed Crib being moved into position, see video below

Basically, stack of timbers fastened together "Lincoln log" style and covered with a flat bottom to spread the load out on the soft soil under the boardwalk. Sometimes, cribs are filled with rocks when used as abutments for trail bridges over streams.

About 800lbs of materials....

To make all this happen, the crib materials are carried to the jobsite--in this case about 1/4 mile from the Shays' parking area.

A good view of the tilt problem

Once all of the materials have been delivered, it's time to get your boots on

Jacking up the east side...

The sagging sections are then jacked up on each side. Because the entire boardwalk is connected, the work needs to take place at multiple points at the same time--otherwise, we'll pull the sections apart. Placing the jacks under the support helps to straighten out the sagging helical piles as the bridge is raised up.

...then the west side.

Boardwalk must be lifted high enough for crib to be set in place underneath it.

Crib Construction

Cribs are built, slid into location, and dumped over the edge, right side up into the mud.

A period of shoving the crib into place under the supports then follows. This is usually accomplished by some mild cursing and grunting as the thing is wiggled through the mud into place.

Crib in place. Water below, boardwalk above.

We installed 4 cribs in all, ready for hikers. We'll see what next winter brings. The video and most of the photos in this post were taken by Christine Ward, the maintainer for this section of the AT.